Localization of VGLUT3, the vesicular glutamate transporter type 3, in the rat brain

Glutamatergic nonpyramidal neurons from neocortical layer VI and their comparison with pyramidal and spiny stellate neurons

The deeper part of neocortical layer VI is dominated by nonpyramidal neurons, which lack a prominent vertically ascending dendrite and predominantly establish corticocortical connections. These neurons were studied in rat neocortical slices using patch-clamp, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling. The majority of these neurons expressed the vesicular glutamate transporter but not glutamic acid decarboxylase, suggesting that a high proportion of layer VI nonpyramidal neurons are glutamatergic. Indeed, they exhibited numerous dendritic spines and established asymmetrical synapses. Our sample of glutamatergic nonpyramidal neurons displayed a wide variety of somatodendritic morphologies and a subset of these cells expressed the Nurr1 mRNA, a marker for ipsilateral, but not commissural corticocortical projection neurons in layer VI. Comparison with spiny stellate and pyramidal neurons from other layers showed that glutamatergic neurons consistently exhibited a low occurrence of GABAergic interneuron markers and regular spiking firing patterns. Analysis of electrophysiological diversity using unsupervised clustering disclosed three groups of cells. Layer V pyramidal neurons were segregated into a first group, whereas a second group consisted of a subpopulation of layer VI neurons exhibiting tonic firing. A third heterogeneous cluster comprised spiny stellate, layer II/III pyramidal, and layer VI neurons exhibiting adaptive firing. The segregation of layer VI neurons in two different clusters did not correlate either with their somatodendritic morphologies or with Nurr1 expression. Our results suggest that electrophysiological similarities between neocortical glutamatergic neurons extend beyond layer positioning, somatodendritic morphology, and projection specificity.

Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum

The lateral septum (LS) plays a role in the adjustment of behavioral responses according to environmental demands. This is a complex integrative process wherein a variety of modulatory systems, i.e. cholinergic, dopaminergic and serotonergic projections forming pericellular baskets around LS neurons, are involved. Recently, vesicular glutamate transporter 3 (VGLUT3)-immunoreactive (-ir) structures outlining unlabeled somata and their proximal dendrites were described in the LS. However, the vesicular transporters for acetylcholine and GABA were not or only rarely co-expressed with VGLUT3. In this study, the morphology and distribution of these VGLUT3-ir structures were systematically analyzed revealing that (1) they form distinct pericellular baskets (PBs) displaying variable shapes, (2) they are arranged in a layer-like pattern similar to the terminals of other modulatory systems, (3) beside a few exceptions (e.g., choline acetyltransferase), they are generally not or very sparsely co-localized with other neurochemical markers characterizing major neuron populations or afferent systems of the LS, i.e. calcium-binding proteins, tyrosine hydroxylase, tryptophan hydroxylase, vesicular glutamate transporters 1 (VGLUT1) and 2 (VGLUT2) and the vesicular GABA transporter. Thus, in the LS, a separate population of neurons is covered by VGLUT3-ir PBs. The distribution pattern and the lack of co-localization indicate that the VGLUT3-expressing cells of origin are located in the brainstem and that they could be pure glutamatergic projection neurons-different from the well-defined canonical VGLUT1- and VGLUT2-expressing neurons. Alternatively, they could simultaneously express VGLUT3 and second transmitter, but use different release sites inside the LS for both.

Impairment of SLC17A8 encoding vesicular glutamate transporter-3, VGLUT3, underlies nonsyndromic deafness DFNA25 and inner hair cell dysfunction in null mice

Autosomal-dominant sensorineural hearing loss is genetically heterogeneous, with a phenotype closely resembling presbycusis, the most common sensory defect associated with aging in humans. We have identified SLC17A8, which encodes the vesicular glutamate transporter-3 (VGLUT3), as the gene responsible for DFNA25, an autosomal-dominant form of progressive, high-frequency nonsyndromic deafness. In two unrelated families, a heterozygous missense mutation, c.632C-->T (p.A211V), was found to segregate with DFNA25 deafness and was not present in 267 controls. Linkage-disequilibrium analysis suggested that the families have a distant common ancestor. The A211 residue is conserved in VGLUT3 across species and in all human VGLUT subtypes (VGLUT1-3), suggesting an important functional role. In the cochlea, VGLUT3 accumulates glutamate in the synaptic vesicles of the sensory inner hair cells (IHCs) before releasing it onto receptors of auditory-nerve terminals. Null mice with a targeted deletion of Slc17a8 exon 2 lacked auditory-nerve responses to acoustic stimuli, although auditory brainstem responses could be elicited by electrical stimuli, and robust otoacoustic emissions were recorded. Ca(2+)-triggered synaptic-vesicle turnover was normal in IHCs of Slc17a8 null mice when probed by membrane capacitance measurements at 2 weeks of age. Later, the number of afferent synapses, spiral ganglion neurons, and lateral efferent endings below sensory IHCs declined. Ribbon synapses remaining by 3 months of age had a normal ultrastructural appearance. We conclude that deafness in Slc17a8-deficient mice is due to a specific defect of vesicular glutamate uptake and release and that VGLUT3 is essential for auditory coding at the IHC synapse.

Enhanced glutamatergic phenotype of mesencephalic dopamine neurons after neonatal 6-hydroxydopamine lesion

There is increasing evidence that a subset of midbrain dopamine (DA) neurons uses glutamate as a co-transmitter and expresses vesicular glutamate transporter (VGLUT) 2, one of the three vesicular glutamate transporters. In the present study, double in situ hybridization was used to examine tyrosine hydroxylase (TH) and VGLUT2 mRNA expression during the embryonic development of these neurons, and postnatally, in normal rats and rats injected with 6-hydroxydopamine (6-OHDA) at P4 to destroy partially DA neurons. At embryonic days 15 and 16, there was a regional overlap in the labeling of TH and VGLUT2 mRNA in the ventral mesencephalon, which was no longer found at late embryonic stages (E18-E21) and postnatally. In normal pups from P5 to P15, only 1-2% of neurons containing TH mRNA in the ventral tegmental area (VTA) and substantia nigra, pars compacta, also displayed VGLUT2 mRNA. In contrast, after the cerebroventricular administration of 6-OHDA at P4, 26% of surviving DA neurons in the VTA of P15 rats expressed VGLUT2. To search for a colocalization of TH and VGLUT2 protein in axon terminals of these neurons, the nucleus accumbens of normal and 6-OHDA-lesioned P15 rats was examined by electron microscopy after dual immunocytochemical labeling. In normal rats, VGLUT2 protein was found in 28% of TH positive axon terminals in the core of nucleus accumbens. In 6-OHDA-lesioned rats, the total number of TH positive terminals was considerably reduced, and yet the proportion also displaying VGLUT2 immunoreactivity was modestly but significantly increased (37%). These results lead to the suggestion that the glutamatergic phenotype of a VTA DA neurons is highly plastic, repressed toward the end of normal embryonic development, and derepressed postnatally following injury. They also support the hypothesis of co-release of glutamate and DA by mesencephalic neurons in vivo, at least in the developing brain.

Developmental and target-dependent regulation of vesicular glutamate transporter expression by dopamine neurons

Mesencephalic dopamine (DA) neurons have been suggested to use glutamate as a cotransmitter. Here, we suggest a mechanism for this form of cotransmission by showing that a subset of DA neurons both in vitro and in vivo expresses vesicular glutamate transporter 2 (VGluT2). Expression of VGluT2 decreases with age. Moreover, when DA neurons are grown in isolation using a microculture system, there is a marked upregulation of VGluT2 expression. We provide evidence that expression of this transporter is normally repressed through a contact-dependent interaction with GABA and other DA neurons, thus providing a partial explanation for the highly restricted expression of VGluT2 in DA neurons in vivo. Our results demonstrate that the neurotransmitter phenotype of DA neurons is both developmentally and dynamically regulated. These findings may have implications for a better understanding of the fast synaptic action of DA neurons as well as basal ganglia circuitry.

Immunolocalization of vesicular glutamate transporters 1 and 2 in the rat inferior colliculus

The inferior colliculus is a major relay nucleus in the ascending auditory pathways that receives multiple glutamatergic inputs. Vesicular glutamate transporters 1 and 2 (VGLUT1, VGLUT2) most often have complementary non-overlapping distributions and can be used to differentiate glutamatergic inputs. The present study therefore examined co-immunolabeling of VGLUT1 and VGLUT2 in three divisions of the rat inferior colliculus. Additional co-immunolabeling of microtubule-associated protein 2 and neuronal class III beta-tubulin provided visualization of neuronal soma and processes and allowed identification of axo-somatic versus axo-dendritic contacts. Results showed numerous VGLUT1 and 2 immunolabeled terminals in the central nucleus, lateral cortex and dorsal cortex. In all three divisions there was little to no co-containment of the two vesicular glutamate transporters indicating a complementary distribution. VGLUT1 made predominantly axo-dendritic connections in the neuropil, while VGLUT2 had many axo-somatic contacts in addition to axo-dendritic contacts. VGLUT2 immunolabeled terminals were numerous on the soma and proximal dendrites of many medium-to-large and large neurons in the central nucleus and medium to large neurons in the dorsal cortex. There were more VGLUT2 terminals than VGLUT1 in all divisions and more VGLUT2 terminals in dorsal and lateral cortices than in the central nucleus. This study shows that VGLUT1 and VGLUT2 differentiate complementary patterns of glutamatergic inputs into the central nucleus, lateral and dorsal cortex of the inferior colliculus with VGLUT1 endings predominantly on the dendrites and VGLUT2 on both dendrites and somas.

Revealing the molecular layer of the primate dorsal cochlear nucleus

In nonprimate mammals, the dorsal cochlear nucleus (DCN) is thought to play a role in the orientation of the head toward sounds of interest by integrating acoustic and somatosensory information. Humans and higher primates might not use this system because of reported phylogenetic changes in DCN cytoarchitecture [Moskowitz N (1969) Comparative aspects of some features of the central auditory system of primates. Ann N Y Acad Sci 167:357-369; Moore JK, Osen KK (1979) The cochlear nuclei in man. Am J Anat 154:393-418; Moore JK (1980) The primate cochlear nuclei: loss of lamination as a phylogenetic process. J Comp Neurol 193:609-629]. In this study, we re-evaluated this question from a comparative perspective and examined the rhesus monkey (cercopithecoid primate) using more sensitive probes and higher resolution imaging methods. We used electron microscopy to identify parallel fibers and their synapses, and molecular markers to determine that primates exhibit the main components of excitatory neurotransmission as other mammals. We observed that characteristics of the monkey molecular layer resembled what has been reported for nonprimates: (1) immunohistochemistry revealed many unmyelinated, thin axons and en passant glutamatergic synapses on dendritic spines; (2) immunohistochemistry for phosphodiesterase (PDE10A) showed the nuclei of granule cells distributed in the external molecular layer and the deep layers in the DCN; (3) antibodies for the inositol trisphosphate receptor (IP3r) and calbindin immunostained cartwheel cells; (4) postembedding immunogold labeling revealed synaptic expression of AMPA and delta glutamate receptor subunits on spines in parallel fiber endings; and (5) parallel fibers use vesicular glutamate transporter 1 (VGLUT1) to package glutamate into the synaptic vesicles and to mediate glutamate transport. These observations are consistent with the argument that the rhesus monkey DCN has neuronal features similar to those of other nonprimate mammals.

Targeting the glutamatergic system to develop novel, improved therapeutics for mood disorders

Mood disorders are common, chronic, recurrent mental illnesses that affect the lives of millions of individuals worldwide. To date, the monoaminergic systems (serotonergic, noradrenergic and dopaminergic) in the brain have received the greatest attention in neurobiological studies of mood disorders, and most therapeutics target these systems. However, there is growing evidence that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in mood-disorder pathophysiology as well as the efficacy of glutamatergic agents in mood disorders. We also discuss exciting new prospects for the development of improved therapeutics for these devastating disorders.

The vesicular glutamate transporter VGLUT3 synergizes striatal acetylcholine tone

Three subtypes of vesicular transporters accumulate glutamate into synaptic vesicles to promote its vesicular release. One of the subtypes, VGLUT3, is expressed in neurons, including cholinergic striatal interneurons, that are known to release other classical transmitters. Here we showed that disruption of the Slc17a8 gene (also known as Vglut3) caused an unexpected hypocholinergic striatal phenotype. Vglut3(-/-) mice were more responsive to cocaine and less prone to haloperidol-induced catalepsy than wild-type littermates, and acetylcholine release was decreased in striatum slices lacking VGLUT3. These phenotypes were associated with a colocalization of VGLUT3 and the vesicular acetylcholine transporter (VAChT) in striatal synaptic vesicles and the loss of a synergistic effect of glutamate on vesicular acetylcholine uptake. We propose that this vesicular synergy between two transmitters is the result of the unbalanced bioenergetics of VAChT, which requires anion co-entry for continuing vesicular filling. Our study reveals a previously unknown effect of glutamate on cholinergic synapses with potential functional and pharmacological implications.

Functional implications of glutamatergic projections to the ventral tegmental area

Glutamatergic afferents of the ventral tegmental area (VTA) play an important role in the functioning of the VTA and are involved in the pathophysiology of drug addiction. It has recently been demonstrated that the VTA is densely innervated by glutamatergic axons and that glutamatergic neurons projecting to the VTA are situated in almost all structures that project there. While the projection from the prefrontal cortex is essentially entirely glutamatergic, subcortical glutamatergic neurons innervating the VTA intermingle with non-glutamatergic, most likely GABAergic and/or peptidergic VTA-projecting neurons. The first part of this review focuses on the origins and putative functional implications of various glutamatergic projections to the VTA. In the second part we consider how different neuropeptides via different mechanisms modulate glutamatergic actions in the VTA. We conclude by developing a model of how the glutamatergic afferents might together contribute to the functions of the VTA.

Two distinct subtypes of serotonergic fibers classified by co-expression with vesicular glutamate transporter 3 in rat forebrain

Although virtually all of serotonin (5-HT) neurons in the midbrain raphe nuclei of rats are known to express vesicular glutamate transporter 3 (VGLUT3), VGLUT3-positive 5-HT fibers have been identified only in the cerebral cortex and hippocampus. Thus, our understanding of forebrain sites where 5-HT-glutamate interaction may be potentially managed by such possible glutamatergic 5-HT fibers themselves, is still largely fragmentary from a morphological point of view. To address this issue, we analyzed the rat forebrain by immunohistochemistry and chemical lesion experiment of 5-HT neurons by intracerebroventricular injection of a neurotoxin, 5,7-dihydroxytriptamine. Contrary to expectation, the double-label immunofluorescence staining revealed that the incidence of VGLUT3-positive 5-HT fibers is generally low over the forebrain, demonstrating occasional fibers with one or two double-labeled varicosities. The most extreme example was the nucleus of the lateral olfactory tract (LOT), which seemed to be devoid of double-labeled fibers despite high densities of 5-HT fibers and VGLUT3-positive fibers. In sharp contrast, robust plexuses of VGLUT3-positive 5-HT fibers were found in the dorsal, but not ventral, part of the lateral septum. The lesion experiment carried out to explore whether VGLUT3 exists in 5-HT fibers showed that in lesioned rats VGLUT3-positive fibers almost completely disappear from the septal region but seemed unchanged in the LOT. The present study shows that midbrain raphe-derived 5-HT fibers can be classified into two subtypes depending on co-expression with VGLUT3 staining in the forebrain.

Co-localization of corticotropin-releasing factor and vesicular glutamate transporters within axon terminals of the rat dorsal raphe nucleus

Electrophysiological, microdialysis and behavioral studies support a modulatory role for corticotropin-releasing factor (CRF) in regulating the dorsal raphe nucleus (DRN)-serotonin (5-HT) system. CRF and 5-HT are implicated in the pathophysiology of depression, thus neuroanatomical substrates of CRF-DRN-5-HT interactions are of interest. Identification of co-transmitters within DRN CRF axon terminals is important for elucidating the complex effects underlying CRF afferent regulation of DRN neurons. This study investigated whether CRF-labeled axon terminals within the DRN contain immunoreactivity for vesicular glutamate transporters (isoforms vGlut1 and vGlut2) indicative of the excitatory neurotransmitter glutamate. Dual immunohistochemistry for CRF and either vGlut1 or vGlut2 was conducted within the same tissue section and immunofluorescence results indicated patterns of immunoreactivity consistent with previous reports. Abundant vGlut1- and vGlut2-immunoreactivity was found in puncta exhibiting a largely uniform distribution, whereas CRF-immunoreactivity was localized to topographically distributed varicose processes within the DRN. Profiles containing both CRF- and either vGlut1- or vGlut2-immunoreactivity were apparent in the DRN. Electron microscopy confirmed that immunoreactivity for CRF and vGlut1 was localized primarily to separate axon terminals in the DRN, with a subset co-localizing CRF and vGlut1. Examination of CRF and vGlut2 immunoreactivities in the DRN indicated that CRF and vGlut2 were found within the same axon terminal more frequently than CRF and vGlut1. Overall, these anatomical findings suggest that CRF may function, in part, with the excitatory neurotransmitter glutamate in the modulation of neuronal activity in the DRN.

Intrinsic excitability of cholinergic neurons in the rat parabigeminal nucleus

Cholinergic neurons in the parabigeminal nucleus of the rat midbrain were studied in an acute slice preparation. Spontaneous, regular action potentials were observed both with cell-attached patch recordings as well as with whole cell current-clamp recordings. The spontaneous activity of parabigeminal nucleus (PBN) neurons was not due to synaptic input as it persisted in the presence of the pan-ionotropic excitatory neurotransmitter receptor blocker, kynurenic acid, and the cholinergic blockers dihydro-beta-erythroidine (DHbetaE) and atropine. This result suggests the existence of intrinsic currents that enable spontaneous activity. In voltage-clamp recordings, I(H) and I(A) currents were observed in most PBN neurons. I(A) had voltage-dependent features that would permit it to contribute to spontaneous firing. In contrast, I(H) was significantly activated at membrane potentials lower than the trough of the spike afterhyperpolarization, suggesting that I(H) does not contribute to spontaneous firing of PBN neurons. Consistent with this interpretation, application of 25 microM ZD-7288, which blocked I(H), did not affect the rate of spontaneous firing in PBN neurons. Counterparts to I(A) and I(H) were observed in current-clamp recordings: I(A) was reflected as a slow voltage ramp observed between action potentials and on release from hyperpolarization, and I(H) was reflected as a depolarizing sag often accompanied by rebound spikes in response to hyperpolarizing current injections. In response to depolarizing current injections, PBN neurons fired at high frequencies, with relatively little accommodation. Ultimately, the spontaneous activity in PBN neurons could be used to modulate cholinergic drive in the superior colliculus in either positive or negative directions.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

Role of glutamate in locomotor rhythm generating neuronal circuitry

Central pattern generators (CPGs) are defined as neuronal circuits capable of producing a rhythmic and coordinated output without the influence of sensory input. The locomotor and respiratory neuronal circuits are two of the better-characterized CPGs, although much work remains to fully understand how these networks operate. Glutamatergic neurons are involved in most neuronal circuits of the nervous system and considerable efforts have been made to study glutamate receptors in nervous system signaling using a variety of approaches. Because of the complexity of glutamate-mediated signaling and the variety of receptors triggered by glutamate, it has been difficult to pinpoint the role of glutamatergic neurons in neuronal circuits. In addition, glutamate is an amino acid used by every cell, which has hampered identification of glutamatergic neurons. Glutamatergic excitatory neurotransmission is dependent on the release from glutamate-filled presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Recent data describe that Vglut2 (Slc17a6) null mutant mice die immediately after birth due to a complete loss of the stable autonomous respiratory rhythm generated by the pre-Bötzinger complex. Surprisingly, we found that basal rhythmic locomotor activity is not affected in Vglut2 null mutant embryos. With this perspective, we discuss data regarding presence of VGLUT1, VGLUT2 and VGLUT3 positive neuronal populations in the spinal cord.

Glutamate synaptic inputs to ventral tegmental area neurons in the rat derive primarily from subcortical sources

Dopamine and GABA neurons in the ventral tegmental area project to the nucleus accumbens and prefrontal cortex and modulate locomotor and reward behaviors as well as cognitive and affective processes. Both midbrain cell types receive synapses from glutamate afferents that provide an essential control of behaviorally-linked activity patterns, although the sources of glutamate inputs have not yet been completely characterized. We used antibodies against the vesicular glutamate transporter subtypes 1 and 2 (VGlut1 and VGlut2) to investigate the morphology and synaptic organization of axons containing these proteins as putative markers of glutamate afferents from cortical versus subcortical sites, respectively, in rats. We also characterized the ventral tegmental area cell populations receiving VGlut1+ or VGlut2+ synapses according to their transmitter phenotype (dopamine or GABA) and major projection target (nucleus accumbens or prefrontal cortex). By light and electron microscopic examination, VGlut2+ as opposed to VGlut1+ axon terminals were more numerous, had a larger average size, synapsed more proximally, and were more likely to form convergent synapses onto the same target. Both axon types formed predominantly asymmetric synapses, although VGlut2+ terminals more often formed synapses with symmetric morphology. No absolute selectivity was observed for VGlut1+ or VGlut2+ axons to target any particular cell population. However, the synapses onto mesoaccumbens neurons more often involved VGlut2+ terminals, whereas mesoprefrontal neurons received relatively equal synaptic inputs from VGlut1+ and VGlut2+ profiles. The distinct morphological features of VGlut1 and VGlut2 positive axons suggest that glutamate inputs from presumed cortical and subcortical sources, respectively, differ in the nature and intensity of their physiological actions on midbrain neurons. More specifically, our findings imply that subcortical glutamate inputs to the ventral tegmental area expressing VGlut2 predominate over cortical sources of excitation expressing VGlut1 and are more likely to drive the behaviorally-linked bursts in dopamine cells that signal future expectancy or attentional shifting.

Loss of VGLUT1 and VGLUT2 in the prefrontal cortex is correlated with cognitive decline in Alzheimer disease

Several lines of evidence suggest that the glutamatergic system is severely impaired in Alzheimer disease (AD). Here, we assessed the status of glutamatergic terminals in AD using the first available specific markers, the vesicular glutamate transporters VGLUT1 and VGLUT2. We quantified VGLUT1 and VGLUT2 in the prefrontal dorsolateral cortex (Brodmann area 9) of controls and AD patients using specific antiserums. A dramatic decrease in VGLUT1 and VGLUT2 was observed in AD using Western blot. Similar decreases were observed in an independent group of subjects using immunoautoradiography. The VGLUT1 reduction was highly correlated with the degree of cognitive impairment, assessed with the clinical dementia rating (CDR) score. A significant albeit weaker correlation was also observed with VGLUT2. These findings provide evidence indicating that glutamatergic systems are severely impaired in the A9 region of AD patients and that this impairment is strongly correlated with the progression of cognitive decline. Our results suggest that VGLUT1 expression in the prefrontal cortex could be used as a valuable neurochemical marker of dementia in AD.

Glutamatergic afferents of the ventral tegmental area in the rat

Glutamatergic inputs to the ventral tegmental area (VTA), thought crucial to the capacity of the VTA to detect and signal stimulus salience, have been reported to arise in but a few structures. However, the afferent system of the VTA comprises very abundant neurons within a large formation extending from the prefrontal cortex to the caudal brainstem. Neurons in nearly all parts of this continuum may be glutamatergic and equivalently important to VTA function. Thus, we sought to identify the full range of glutamatergic inputs to the VTA by combining retrograde transport of wheat germ agglutinin-bound gold after injections into the VTA with nonisotopic in situ hybridization of the vesicular glutamate transporters (VGLUTs) 1, 2, and 3. We found glutamatergic neurons innervating the VTA in almost all structures projecting there and that a majority of these are subcortical and VGLUT2 mRNA positive. The tremendous convergence of glutamatergic afferents from many brain areas in the VTA suggests that (1) the function of the VTA requires integration of manifold and diverse bits of information and (2) the activity of the VTA reflects the ongoing activities of various combinations of its afferents.

Glutamatergic functions of primary afferent neurons with special emphasis on vagal afferents

Glutamate has been identified as the main transmitter of primary afferent neurons. This was established based on biochemical, electrophysiological, and immunohistochemical data from studies on glutamatergic receptors and their agonists/antagonists. The availability of specific antibodies directed against glutamate and, more recently, vesicular glutamate transporters corroborated this and led to significant new discoveries. In particular, peripheral endings of various classes of afferents contain vesicular glutamate transporters, suggesting vesicular storage in and exocytotic release of glutamate from peripheral afferent endings. This suggests that autocrine mechanisms regulate sensory transduction processes. However, glutamate release from peripheral sensory terminals could also enable afferent neurons to influence various cells associated with them. This may be particularly relevant for vagal intraganglionic laminar endings, which could represent glutamatergic sensor-effector components of intramural reflex arcs in the gastrointestinal tract. Thus, morphological analysis of the relationships of putative glutamatergic primary afferents with associated tissues may direct forthcoming studies on their functions.

Enhanced anxiety, depressive-like behaviour and impaired recognition memory in mice with reduced expression of the vesicular glutamate transporter 1 (VGLUT1)

Three isoforms of a vesicular glutamate transporter (VGLUT1-3) have been identified. Of these, VGLUT1 is the major isoform in the cerebral cortex and hippocampus where it is selectively located on synaptic vesicles of excitatory glutamatergic terminals. Variations in VGLUT1 expression levels have a major impact on the efficacy of glutamate synaptic transmission. Given evidence linking alterations in glutamate neurotransmission to various neuropsychiatric disorders, we investigated the possible influence of a down-regulation of VGLUT1 transporter on anxiety, depressive-like behaviour and learning. The behavioural phenotype of VGLUT1-heterozygous mice (C57BL/6) was compared to wild-type (WT) littermates. Moreover, VGLUT1-3 expression, hippocampal excitatory terminal ultrastructure and neurochemical phenotype were analysed. VGLUT1-heterozygous mice displayed normal spontaneous locomotor activity, increased anxiety in the light-dark exploration test and depressive-like behaviour in the forced swimming test: no differences were shown in the elevated plus-maze model of anxiety. In the novel object recognition test, VGLUT1(+/-) mice showed normal short-term but impaired long-term memory. Spatial memory in the Morris water maze was unaffected. Western blot analysis confirmed that VGLUT1 heterozygotes expressed half the amount of transporter compared to WT. In addition, a reduction in the reserve pool of synaptic vesicles of hippocampal excitatory terminals and a 35-45% reduction in GABA in the frontal cortex and the hippocampus were observed in the mutant mice. These observations suggest that a VGLUT1-mediated presynaptic alteration of the glutamatergic synapses, in specific brain regions, leads to a behavioural phenotype resembling certain aspects of psychiatric and cognitive disorders.

Synapse-specific localization of vesicular glutamate transporters in the rat olfactory bulb

Vesicular glutamate transporters (VGLUTs) mediate the packaging of the excitatory neurotransmitter glutamate into synaptic vesicles. Three VGLUT subtypes have so far been identified, with distinct expression patterns in the adult brain. Here, we investigated the spatial distribution of the three VGLUTs in the rat olfactory bulb, a brain region containing a variety of glutamate synapses, both axodendritic and dendrodendritic. Using multilabelling confocal microscopy and electron microscopic immunocytochemistry, we showed that each VGLUT isoform has a highly selective localization in olfactory bulb synapses. VGLUT1 is present at dendrodendritic synapses established by the output neurones (mitral and tufted cells) with bulbar interneurones in the glomerular layer and external plexiform layer, as well as in axonal synapses of the granule cell layer. By contrast, VGLUT2 is strongly expressed in axon terminals of olfactory sensory neurones, which establish synapses with second-order neurones in the glomerular neuropil. VGLUT2 is also found in the outer part of the external plexiform layer and in the granule cell layer but colocalizes only partially with VGLUT1. Finally, we showed that VGLUT3 is exclusively located in the glomerular neuropil, where it colocalizes extensively with the vesicular inhibitory amino acid transporter vesicular GABA transporter, suggesting that it is associated with a subset of inhibitory synapses. Together, these observations extend previous findings on VGLUT distribution in the forebrain, and suggest that each VGLUT subtype has a specific function in the distinct features of axodendritic and dendrodendritic synapses that characterize the olfactory bulb circuit.

The ability of the mesocortical dopamine system to operate in distinct temporal modes

BACKGROUND

This review discusses evidence that cells in the mesocortical dopamine (DA) system influence information processing in target areas across three distinct temporal domains.

DISCUSSIONS

Phasic bursting of midbrain DA neurons may provide temporally precise information about the mismatch between expected and actual rewards (prediction errors) that has been hypothesized to serve as a learning signal in efferent regions. However, because DA acts as a relatively slow modulator of cortical neurotransmission, it is unclear whether DA can indeed act to precisely transmit prediction errors to prefrontal cortex (PFC). In light of recent physiological and anatomical evidence, we propose that corelease of glutamate from DA and/or non-DA neurons in the VTA could serve to transmit this temporally precise signal. In contrast, DA acts in a protracted manner to provide spatially and temporally diffuse modulation of PFC pyramidal neurons and interneurons. This modulation occurs first via a relatively rapid depolarization of fast-spiking interneurons that acts on the order of seconds. This is followed by a more protracted modulation of a variety of other ionic currents on timescales of minutes to hours, which may bias the manner in which cortical networks process information. However, the prolonged actions of DA may be curtailed by counteracting influences, which likely include opposing actions at D1 and D2-like receptors that have been shown to be time- and concentration-dependent. In this way, the mesocortical DA system optimizes the characteristics of glutamate, GABA, and DA neurotransmission both within the midbrain and cortex to communicate temporally precise information and to modulate network activity patterns on prolonged timescales.

Vessicular glutamate transporters 1 and 2 are differentially associated with auditory nerve and spinal trigeminal inputs to the cochlear nucleus

Projections of glutamatergic somatosensory and auditory fibers to the cochlear nucleus (CN) are mostly nonoverlapping: projections from the spinal trigeminal nucleus (Sp5) terminate primarily in the granule cell domains (GCD) of CN, whereas type I auditory nerve fibers (ANFs) project to the magnocellular areas of the VCN (VCNm) and deep layers of Dorsal CN (DCN). Vesicular glutamate transporters (VGLUTs), which selectively package glutamate into synaptic vesicles, have different isoforms associated with distinct subtypes of excitatory glutamatergic neurons. Here we examined the distributions of VGLUT1 and VGLU2 expression in the CN and their colocalization with Sp5 and ANF terminals following injections of anterograde tracers into Sp5 and the cochlea in the guinea pig. The CN regions that showed the most intense expression of VGLUT1 and VGLUT2 were largely nonoverlapping and were consistent with ANF and Sp5 projections, respectively: VGLUT1 was highly expressed in VCNm and the molecular layer of the DCN, whereas VGLUT2 was expressed predominantly in the GCD. Half (47% +/- 3%) of the Sp5 mossy fiber endings colabeled with VGLUT2, but few (2.5% +/- 1%) colabeled with VGLUT1. In contrast, ANFs colabeled predominantly with VGLUT1. The pathway-specific expression of VGLUT isoforms in the CN may be associated with the intrinsic synaptic properties that are unique to each sensory pathway.

CB1 receptors diminish both Ca(2+) influx and glutamate release through two different mechanisms active in distinct populations of cerebrocortical nerve terminals

We have investigated the mechanisms by which activation of cannabinoid receptors reduces glutamate release from cerebrocortical nerve terminals. Glutamate release evoked by depolarization of nerve terminals with high KCl (30 mmol/L) involves N and P/Q type Ca(2+)channel activation. However, this release of glutamate is independent of Na(+) or K(+) channel activation as it was unaffected by blockers of these channels (tetrodotoxin -TTX- or tetraethylammonium TEA). Under these conditions in which only Ca(2+) channels contribute to pre-synaptic activity, the activation of cannabinoid receptors with WIN55,212-2 moderately reduced glutamate release (26.4 +/- 1.2%) by a mechanism that in this in vitro model is resistant to TTX and consistent with the inhibition of Ca(2+) channels. However, when nerve terminals are stimulated with low KCl concentrations (5-10 mmol/L) glutamate release is affected by both Ca(2+) antagonists and also by TTX and TEA, indicating the participation of Na(+) and K(+) channel firing in addition to Ca(2+) channel activation. Interestingly, stimulation of nerve terminals with low KCl concentrations uncovered a mechanism that further inhibited glutamate release (81.78 +/- 4.9%) and that was fully reversed by TEA. This additional mechanism is TTX-sensitive and consistent with the activation of K(+) channels. Furthermore, Ca(2+) imaging of single boutons demonstrated that the two pre-synaptic mechanisms by which cannabinoid receptors reduce glutamate release operate in distinct populations of nerve terminals.

Vesicular glutamate transporters VGLUT1 and VGLUT2 in the developing mouse barrel cortex

Three vesicular glutamate transporters have been identified in mammals. Two of them, VGLUT1 and VGLUT2, define the glutamatergic phenotype and their distribution in the brain is almost complementary. In the present study we examined the distribution and expression levels of these two VGLUTs during postnatal development of the mouse barrel cortex. We also investigated changes in the localization of VGLUT1 and VGLUT2 within particular compartments of the barrel field (barrels/septa) during its development. We found differences in the time course of developmental expression, with VGLUT1 peaking around P14, while VGLUT2 increased gradually until adulthood. Over the examined period (P3 - adult) both transporters had stronger expression in the barrel interiors, and in this compartment VGLUT2 dominated, whereas in the inter-barrel septa VGLUT1 dominated over VGLUT2. Furthermore, we found that some nerve terminals in the barrel cortex coexpressed both transporters until adulthood. Colocalization was observed within the barrels, but not within the septa.

Extrinsic afferent systems to the dentate gyrus

The dentate gyrus is the first stage of the intrahippocampal, excitatory, trisynaptic loop, and a primary target of the majority of entorhinal afferents that terminate in a laminar fashion on granule cell dendrites and carry sensory information of multiple modalities about the external world. The electric activity of the trisynaptic pathway is controlled mainly by different types of local, GABAergic interneurons, and subcortical and commissural afferents. In this chapter we will outline the origin and postsynaptic targets in the dentate gyrus of chemically identified subcortical inputs. These systems are afferents originating from the medial septum/diagonal band of Broca GABAergic and cholinergic neurons, neurochemically distinct types of neurons located in the supramammillary area, serotonergic fibers from the median raphe, noradrenergic afferents from the pontine nucleus, locus ceruleus, dopamine axons originating in the ventral tegmental area, and the commissural projection system. Because of the physiological implications, these afferents are discussed in the context of the glutamatergic innervation of the dentate gyrus. One common feature of the extrinsic dentate afferent systems is that they originate from a relatively small number of neurons. However, the majority of these afferents are able to exert a powerful control over the electrical activity of the hippocampus. This strong influence is due to the fact that the majority of the extrinsic afferents terminate on a relatively small, but specific, populations of neurons that are able to control large areas of the hippocampal formation.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Stereological estimates of the basal forebrain cell population in the rat, including neurons containing choline acetyltransferase, glutamic acid decarboxylase or phosphate-activated glutaminase and colocalizing vesicular glutamate transporters

The basal forebrain (BF) plays an important role in modulating cortical activity and influencing attention, learning and memory. These activities are fulfilled importantly yet not entirely by cholinergic neurons. Noncholinergic neurons also contribute and comprise GABAergic neurons and other possibly glutamatergic neurons. The aim of the present study was to estimate the total number of cells in the BF of the rat and the proportions of that total represented by cholinergic, GABAergic and glutamatergic neurons. For this purpose, cells were counted using unbiased stereological methods within the medial septum, diagonal band, magnocellular preoptic nucleus, substantia innominata and globus pallidus in sections stained for Nissl substance and/or the neurotransmitter enzymes, choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD) or phosphate-activated glutaminase (PAG). In Nissl-stained sections, the total number of neurons in the BF was estimated as approximately 355,000 and the numbers of ChAT-immuno-positive (+) as approximately 22,000, GAD+ approximately 119,000 and PAG+ approximately 316,000, corresponding to approximately 5%, approximately 35% and approximately 90% of the total. Thus, of the large population of BF neurons, only a small proportion has the capacity to synthesize acetylcholine (ACh), one third to synthesize GABA and the vast majority to synthesize glutamate (Glu). Moreover, through the presence of PAG, a proportion of ACh- and GABA-synthesizing neurons also has the capacity to synthesize Glu. In sections dual fluorescent immunostained for vesicular transporters, vesicular glutamate transporter (VGluT) 3 and not VGluT2 was present in the cell bodies of most PAG+ and ChAT+ and half the GAD+ cells. Given previous results showing that VGluT2 and not VGluT3 was present in BF axon terminals and not colocalized with VAChT or VGAT, we conclude that the BF cell population influences cortical and subcortical regions through neurons which release ACh, GABA or Glu from their terminals but which in part can also synthesize and release Glu from their soma or dendrites.

Afferent and efferent connections of the rat retrotrapezoid nucleus

The rat retrotrapezoid nucleus (RTN) contains candidate central chemoreceptors that have extensive dendrites within the marginal layer (ML). This study describes the axonal projections of RTN neurons and their probable synaptic inputs. The ML showed a dense plexus of nerve terminals immunoreactive (ir) for markers of glutamatergic (vesicular glutamate transporters VGLUT1-3), gamma-aminobutyric acid (GABA)-ergic, adrenergic, serotonergic, cholinergic, and peptidergic transmission. The density of VGLUT3-ir terminals tracked the location of RTN chemoreceptors. The efferent and afferent projections of RTN were studied by placing small iontophoretic injections of anterograde (biotinylated dextran amine; BDA) and retrograde (cholera toxin B) tracers where RTN chemoreceptors have been previously recorded. BDA did not label the nearby C1 cells. BDA-ir varicosities were found in the solitary tract nucleus (NTS), all ventral respiratory column (VRC) subdivisions, A5 noradrenergic area, parabrachial complex, and spinal cord. In each target region, a large percentage of the BDA-ir varicosities was VGLUT2-ir (41-83%). Putative afferent input to RTN originated from spinal cord, caudal NTS, area postrema, VRC, dorsolateral pons, raphe nuclei, lateral hypothalamus, central amygdala, and insular cortex. The results suggest that 1) whether or not the ML is specialized for CO(2) sensing, its complex neuropil likely regulates the activity of RTN chemosensitive neurons; 2) the catecholaminergic, cholinergic, and serotonergic innervation of RTN represents a possible substrate for the known state-dependent control of RTN chemoreceptors; 3) VGLUT3-ir terminals are a probable marker of RTN; and 4) the chemosensitive neurons of RTN may provide a chemical drive to multiple respiratory outflows, insofar as RTN innervates the entire VRC.

Glutamate-dopamine cotransmission and reward processing in addiction

While Dale's principle of "one neuron, one neurotransmitter" has undergone revisions to incorporate evidence of the corelease of atypical neurotransmitters such as neuropeptides, the corelease of classical neurotransmitters has only recently been realized. Surprisingly, numerous studies now indicate that the corelease of neurotransmitters in the mammalian central nervous system is not an obscure and rare phenomenon but is widespread and involves most classical neurotransmitters systems. However, the suggestion that glutamate can be coreleased with dopamine (DA) has remained controversial. Furthermore, glutamate-DA cotransmission has not yet been seriously considered in the context of the neurocircuitry of addiction. If glutamate is in fact coreleased with DA as some evidence now suggests, this may have significant implications for advancing our understanding of the interactive role that these 2 neurotransmitters play in cognitive and reward processes. In this commentary, we review the evidence for and against glutamate as a cotransmitter and discuss the potential role of glutamate-DA corelease in addiction. In particular, we describe a recently proposed model in which coreleased glutamate transmits a temporally precise prediction error signal of reward described by Schultz et al., whereas the function of coreleased DA is to exert prolonged modulatory influences on neuronal activity. In addition, we suggest that as alcohol consumption transitions from recreational use to addiction, there is a corresponding transition in the reward valence signal from better than predicted to worse than predicted.

Down-regulation of vesicular glutamate transporters precedes cell loss and pathology in Alzheimer's disease

Alzheimer's disease (AD) is characterized pathologically by plaques, tangles, and cell and synapse loss. As glutamate is the principle excitatory neurotransmitter of the CNS, the glutamatergic system may play an important role in AD. An essential step in glutamate neurotransmission is the concentration of glutamate into synaptic vesicles before release from the presynaptic terminal. Recently a group of proteins responsible for uptake has been identified - the vesicular glutamate transporters (VGLUTs). The generation of antibodies has facilitated the study of glutamatergic neurones. Here, we used antibodies to the VGLUTs together with immunohistochemistry and western blotting to investigate the status of glutamatergic neurones in temporal, parietal and occipital cortices of patients with AD; these regions were chosen to represent severely, moderately and mildly affected regions at the end stage of the disease. There was no change in expression of the synaptic markers in relation to total protein in the temporal cortex, but a significant reduction in synaptophysin and VGLUT1 was found in both the parietal and occipital cortices. These changes were found to relate to the number of tangles in the temporal cortex. There were no correlations with either mental test score or behaviour syndromes, with the exception of depression.

VGLUTs: 'exciting' times for glutamatergic research?

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system (CNS). Glutamate is first synthesized in the cytoplasm of presynaptic terminals before being loaded into synaptic vesicles, which fuse with the plasma membrane, releasing their contents, in response to neuronal activity. The important process of synaptic vesicle loading is mediated by a transport protein, collectively known as vesicular glutamate transporter (VGLUT). Controlling the activity of these transporters could potentially modulate the efficacy of glutamatergic neurotransmission. In recent years, three isoforms of mammalian VGLUTs have been cloned and molecularly characterized in detail. Probing these three VGLUTs has been proven to be the most reliable way of visualizing sites of glutamate release in the mammalian CNS. Immunohistochemical studies on VGLUTs suggest that glutamatergic neurons are categorized into subgroups depending on which VGLUT isoform they contain. Recent studies on VGLUT1-deficient mice have led various models to be postulated concerning the possible roles of VGLUTs in synaptic physiology, such as presynaptic regulation of quantal size and activity-dependent short-term plasticity.

Vesicular glutamate (VGlut), GABA (VGAT), and acetylcholine (VACht) transporters in basal forebrain axon terminals innervating the lateral hypothalamus

The basal forebrain (BF) is known to play important roles in cortical activation and sleep, which are likely mediated by chemically differentiated cell groups including cholinergic, gamma-aminobutyric acid (GABA)ergic and other unidentified neurons. One important target of these cells is the lateral hypothalamus (LH), which is critical for arousal and the maintenance of wakefulness. To determine whether chemically specific BF neurons provide an innervation to the LH, we employed anterograde transport of 10,000 MW biotinylated dextran amine (BDA) together with immunohistochemical staining of the vesicular transporter proteins (VTPs) for glutamate (VGluT1, -2, and -3), GABA (VGAT), or acetylcholine (ACh, VAChT). In addition, we applied triple staining for the postsynaptic proteins (PSPs), PSD-95 with VGluT or Gephyrin (Geph) with VGAT, to examine whether the BDA-labeled varicosities may form excitatory or inhibitory synapses in the LH. Axons originating from BDA-labeled neurons in the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) descended within the medial forebrain bundle and extended collateral varicose fibers to contact LH neurons. In the LH, the BDA-labeled varicosities were immunopositive (+) for VAChT ( approximately 10%), VGluT2 ( approximately 25%), or VGAT ( approximately 50%), revealing an important influence of newly identified glutamatergic together with GABAergic BF inputs. Moreover, in confocal microscopy, VGluT2+ and VGAT+ terminals were apposed to PSD-95+ and Geph+ profiles respectively, indicating that they formed synaptic contacts with LH neurons. The important inputs from glutamatergic and GABAergic BF cells could thus regulate LH neurons in an opposing manner to stimulate vs. suppress cortical activation and behavioral arousal reciprocally.

Absence of synapsin I and II is accompanied by decreases in vesicular transport of specific neurotransmitters

Studies of synapsin-deficient mice have shown decreases in the number of synaptic vesicles but knowledge about the consequences of this decrease, and which classes of vesicles are being affected, has been lacking. In this study, glutamatergic, GABAergic and dopaminergic transport has been analysed in animals where the genes encoding synapsin I and II were inactivated. The levels of the vesicular glutamate transporter (VGLUT) 1, VGLUT2 and the vesicular GABA transporter (VGAT) were decreased by approximately 40% in adult forebrain from mice devoid of synapsin I and II, while vesicular monoamine transporter (VMAT) 2 and VGLUT3 were present in unchanged amounts compared with wild-type mice. Functional studies on synaptic vesicles showed that the vesicular uptake of glutamate and GABA was decreased by 41 and 23%, respectively, while uptake of dopamine was unaffected by the lack of synapsin I and II. Double-labelling studies showed that VGLUT1 and VGLUT2 colocalized fully with synapsin I and/or II in the hippocampus and neostriatum, respectively. VGAT showed partial colocalization, while VGLUT3 and VMAT2 did not colocalize with either synapsin I or II in the brain areas studied. In conclusion, distinct vesicular transporters show a variable degree of colocalization with synapsin proteins and, hence, distinct sensitivities to inactivation of the genes encoding synapsin I and II.

Vesicular glutamate transporter 3 (VGLUT3) identifies spatially segregated excitatory terminals in the rat substantia nigra

The excitability of dopaminergic (DA) neurons in the substantia nigra is controlled by the convergent activity of multiple glutamatergic afferents. Here, we show that vesicular glutamate transporter 3 (VGLUT3)-immunoreactive (ir) terminals segregate to the perisomatic region of DA neurons in the substantia nigra pars compacta, and VGLUT3 decorates a synapse population distinct from those marked by vesicular glutamate transporters 1 and 2. VGLUT3-ir nerve endings form asymmetric terminals on DA neurons. Retrograde tracing suggests the superior colliculus as an origin of excitatory VGLUT3-ir afferents. Collectively, our data indicate that VGLUT3 identifies a novel excitatory terminal subset that contributes to the tuning of DA cell excitability in the substantia nigra.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Interaction between the vesicular glutamate transporter type 1 and endophilin A1, a protein essential for endocytosis

In the nerve terminal, neurotransmitter is actively packaged into synaptic vesicles before its release by Ca2+-dependent exocytosis. The three vesicular glutamate transporters (VGLUT1, -2 and -3) are highly conserved proteins that display similar bioenergetic and pharmacological properties but are expressed in different brain areas. We used the divergent C-terminus of VGLUT1 as a bait in a yeast two-hybrid screen to identify and map the interaction between a proline-rich domain of VGLUT1 and the Src homology domain 3 (SH3) domain of endophilin. We further confirmed this interaction by using different glutathione-S-transferase-endophilin fusion proteins to pull down VGLUT1 from rat brain extracts. The expression profiles of the two genes and proteins were compared on rat brain sections, showing that endophilin is most highly expressed in regions and cells expressing VGLUT1. Double immunofluorescence in the rat cerebellum shows that most VGLUT1-positive terminals co-express endophilin, whereas VGLUT2-expressing terminals are often devoid of endophilin. However, neither VGLUT1 transport activity, endophilin enzymatic activity nor VGLUT1 synaptic targeting were altered by this interaction. Overall, the discovery of endophilin as a partner for VGLUT1 in nerve terminals strongly suggests the existence of functional differences between VGLUT1 and -2 terminals in their abilities to replenish vesicle pools.

Altered expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in Parkinson disease

Glutamatergic pathways play a key role in the functional organization of neuronal circuits involved in Parkinson disease (PD). Recently, three vesicular glutamate transporters (VGLUT1-3) were identified. VGLUT1 and VGLUT2 are responsible for the uploading of glutamate into synaptic vesicles and are the first specific markers of glutamatergic neurons available. Here, we analyzed the expression of VGLUT1 and VGLUT2 in autopsy tissues of PD patients and matched controls using Western blot and immunoautoradiography. VGLUT1 and VGLUT2 expression was increased in the Parkinsonian putamen by 24% and 29%, respectively (p<0.01). In contrast, only VGLUT1 was dramatically decreased in the prefrontal and temporal cortex of PD patients (approximately 50%, p<0.01 and p<0.001, respectively). These findings demonstrate the existence of profound alterations of glutamatergic transmission in PD, which are likely to contribute to the motor and cognitive impairments associated with the disease, and should thus be taken into account in the treatment of PD.

Mesocortical dopamine neurons operate in distinct temporal domains using multimodal signaling

In vivo extracellular recording studies have traditionally shown that dopamine (DA) transiently inhibits prefrontal cortex (PFC) neurons, yet recent biophysical measurements in vitro indicate that DA enhances the evoked excitability of PFC neurons for prolonged periods. Moreover, although DA neurons apparently encode stimulus salience by transient alterations in firing, the temporal properties of the PFC DA signal associated with various behaviors is often extraordinarily prolonged. The present study used in vivo electrophysiological and electrochemical measures to show that the mesocortical system produces a fast non-DA-mediated postsynaptic response in the PFC that appears to be initiated by glutamate. In contrast, short burst stimulation of mesocortical DA neurons that produced transient (<4 s) DA release in the PFC caused a simultaneous reduction in spontaneous firing (consistent with extracellular in vivo recordings) and a form of DA-induced potentiation in which evoked firing was increased for tens of minutes (consistent with in vitro measurements). We suggest that the mesocortical system might transmit fast signals about reward or salience via corelease of glutamate, whereas the simultaneous prolonged DA-mediated modulation of firing biases the long-term processing dynamics of PFC networks.

Characterization of medial septal glutamatergic neurons and their projection to the hippocampus

The two neuronal populations that have been typically investigated in the septum use acetylcholine and GABA as neurotransmitters. The existence of noncholinergic, non-GABAergic, most likely glutamatergic septal neurons has recently been reported. However, their morphological characteristics, numbers, distribution, and connectivity have not been determined. Furthermore, the projection of septal glutamatergic neurons to the hippocampus has not been characterized. To address these issues, subpopulations of cholinergic and GABAergic neurons were identified by immunohistochemistry. In addition, the retrograde tracer fluorogold was injected into the hippocampus to determine the characteristics of a glutamatergic septo-hippocampal projection. Our work revealed that although glutamatergic neurons are found throughout the septum, they concentrate in medial septal regions. Using stereological probes, approximately 16,000 glutamatergic neurons were estimated in the medial septal region. Triple immunostaining showed that most glutamatergic neurons do not immunoreact with cholinergic or GABAergic neuronal markers (anti-ChAT or anti-GAD67 antibodies, respectively). Fluorogold injections into CA1, CA3, and dentate gyrus of the hippocampus showed that septal glutamatergic neurons project to each of these hippocampal regions, forming approximately 23% of the septo-hippocampal projection. Most cell bodies of septo-hippocampal glutamatergic neurons were located in the medial septum. The remaining cell bodies were found in the diagonal band. This data shows that glutamatergic neurons constitute a significant neuronal population in the septum and that a subpopulation of these neurons projects to hippocampal regions. Thus, the septo-hippocampal projection needs to be reconsidered as a three neurotransmitter pathway.

Serotonergic and glutamatergic neurons at the ventral medullary surface of the human infant: Observations relevant to central chemosensitivity in early human life

Central chemoreception is the mechanism by which the brain detects the level of carbon dioxide (CO(2)) in the arterial blood and alters breathing accordingly in order to maintain it within physiological levels. The ventral surface of the medulla oblongata (VMS) of animals has long been recognized as a site of chemosensitivity, culminating in the recent identification of chemosensitive serotonergic (5-HT) and glutamatergic (Glut) neurons in this region. In this study, we analyzed the distribution of 5-HT and Glut neurons and their receptors in the arcuate nucleus (Arc) at the VMS of the human infant, using single-and double-label immunohistochemistry with specific antibodies. We also examined the expression of astrocytes, as experimental evidence suggests that astrocytes mediate, at least in part, central chemosensitivity via 5-HT and/or Glut receptors. We identified a small number of 5-HT neurons (approximately 5% of Arc neurons), distributed over the entire extent of the VMS, a large number of Glut neurons (approximately 95% of Arc neurons) that localized almost exclusively to the medial Arc, and a large number of astrocytes distributed across the entire extent of the VMS. The Arc also contained 5-HT(1A), kainate (GluR5), and 5-HT(2A) receptors, which localized predominantly to 5-HT neurons, glutamate neurons and astrocytes, respectively. Astrocytes also expressed the vesicular glutamate transporter 2 and low levels of 5-HT(1A) and kainate (GluR5) receptors, indicating that astrocytes may store and release glutamate, possibly in response to stimulation by 5-HT and/or Glut. These observations suggest that important functional interactions exist between 5-HT, glutamate, and astrocytes in the Arc. They also support the idea that the Arc is homologous to chemosensitive zones at the VMS in experimental animals. These data are important towards delineating the role of the human Arc in modulation of homeostasis, and its dysfunction in brainstem-associated pathologies such as the sudden infant death syndrome.

Changes in purinergic signaling after cerebral injury -- involvement of glutamatergic mechanisms?

Extracellular purines act as neuromodulators on transmitter release and may exert toxic effects at higher concentrations. In microdialysis studies, endogenous ATP facilitated the extracellular concentration of glutamate in the nucleus accumbens (NAc) of rats. Additionally, P2 receptors are involved in astrogliosis in vivo after a stab wound injury in the same region, suggesting that these receptors, preferentially the metabotropic P2Y(1) receptor subtype, mediate also trophic responses. Two sets of experimental findings support the involvement of purinergic and glutamatergic mechanisms in the response of brain to mechanical damage. First, in the present studies, the initial time course of extracellular ATP and glutamate was analyzed after a mechanical injury. The concentration of ATP in microdialysates was elevated only in the first 15-min sample whereas glutamate returned to a basal concentration not before a 90-min period had elapsed. We suggest, that the acute injury-evoked stimulation of P2 receptors contributes to glutamate-mediated excitotoxicity. Second, the expression of P2Y(1) receptors and their possible relation to glutamatergic structures, identified by neuronal vesicular glutamate transporters (VGLUTs), were elucidated in non-treated and mechanically injured animals after 4 days. The number of P2Y(1)-positive cells was significantly increased after injury. Furthermore, P2Y(1) receptor-labeled cells do not exhibit immunoreactivity for VGLUT1 and VGLUT2 without and after injury. However, after injury, a co-expression of the P2Y(1) receptor on VGLUT3-immunopositive cells in the NAc was observed. No VGLUT1-, 2- and 3-immunoreactivity was found on P2Y(1)-positive glial fibrillary acidic protein-immunopositive astrocytes at both conditions. Our data suggest that the expression of P2Y(1) receptors at neurons and astrocytes is modulated in response to cerebral injury. It can be assumed, that the enhanced sensitivity of neurons to purinergic signaling may be related directly or indirectly to changes of the glutamatergic transmission.

Vesicular glutamate transporter 3 immunoreactivity is present in cholinergic basal forebrain neurons projecting to the basolateral amygdala in rat

The basal forebrain (BF) plays a role in behavioral and cortical arousal, attention, learning, and memory. It has been suggested that cholinergic BF neurons co-release glutamate, and some cholinergic BF neurons have been reported to contain vesicular glutamate transporter 3 (VGLUT3). We examined the distribution and projections of BF cholinergic neurons containing VGLUT3, by using dual-label immunofluorescence for choline acetyltransferase (ChAT) and VGLUT3, in situ hybridization, and retrograde tracing. Neurons immunoreactive (+) or containing mRNAs for both ChAT and VGLUT3 were mainly localized to the ventral pallidum and more caudal BF regions; the co-immunoreactive neurons represented 31% of cholinergic neurons in the ventral pallidum and 5-9% more caudally. Examination of cholinergic axon terminals in known target areas of BF projections indicated that the basolateral amygdaloid nucleus contained numerous terminals co-immunoreactive for ChAT and VGLUT3, whereas sampled areas of the olfactory bulb, neocortex, hippocampus, reticular thalamic nucleus, and interpeduncular nucleus were devoid of double-labeled terminals. The basolateral amygdala is innervated by cholinergic BF neurons lacking low-affinity p75 nerve growth factor receptors; many ChAT+VGLUT3+ BF neurons were immunonegative to this receptor. Twenty-five to 79% of ChAT+VGLUT3+ neurons in different BF regions were retrogradely labeled from the basolateral amygdala, up to 52% (ventral pallidum) of the retrogradely labeled ChAT+ neurons were VGLUT3+, and the largest number of amygdala-projecting ChAT+VGluT3+ neurons was found in the ventral pallidum. These findings indicate that BF cholinergic neurons containing VGLUT3 project to the basolateral amygdala and suggest that these neurons might have the capacity to release both acetylcholine and glutamate.

Particular subpopulations of midbrain and hypothalamic dopamine neurons express vesicular glutamate transporter 2 in the rat brain

Vesicular glutamate transporters (VGLUT1, -2, and -3) mediate the accumulation of transmitter glutamate into synaptic vesicles in glutamatergic neurons. VGLUT1 and VGLUT2 are more reliable glutamatergic neuron markers, since VGLUT3 also exists in other neuron types. To study whether the dopaminergic neuron uses glutamate as a cotransmitter, we analyzed VGLUTs expression in dopamine neurons of adult male rats by in situ hybridization and immunohistochemistry. In the ventral midbrain, in situ hybridization analysis revealed no VGLUT1 mRNA expression, a widespread but discrete pattern of VGLUT2 mRNA expression, and a highly limited expression of VGLUT3 mRNA. Reverse-transcriptase polymerase chain reaction analysis detected full-length VGLUT2 gene transcripts in the ventral midbrain. Using in situ hybridization combined with tyrosine hydroxylase (TH) immunostaining, only VGLUT2 signals were detectable in some TH-labeled neurons of A10 dopamine neuron groups, with the highest incidence (20%) in the rostral linear nucleus of the ventral tegmental area. In the forebrain, VGLUT2 signals were demonstrated in half of the A11 TH-labeled neurons in the hypothalamus. Double-label immunostaining for VGLUT2 and vesicular monoamine transporter 2 or TH showed that double-labeled varicosities are rarely observed in any target regions examined of A10 and A11 dopamine neuron groups. These results indicate that VGLUT2 is expressed in subsets of A10 and A11 dopamine neurons, which might release dopamine and glutamate separately from different varicosities in the majority of their single axons.

The neuropathology of autism: a review

Presented is a review of recent progress in the understanding of autism based on investigations of donated human brain tissue. Autism is a pervasive developmental disorder by the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) criteria, manifesting by age 3 and characterized by impairments in social interaction and communication, as well as restricted, repetitive, stereotyped patterns of behavior. Based on reported neuropathologic findings, these characteristic behaviors are clinical manifestations of both pre- and postnatal alterations. This review summarizes the current data obtained from postmortem brain studies in the areas of stereology, neurotransmitter systems/synaptic processes, molecular mechanisms, and neuroimmunology. In addition, we discuss current research strategies designed to facilitate translational research and maximize the yield of precious resources (e.g. the Autism Tissue Program), highlight barriers to research, and consider future trends.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Colocalization of serotonin and vesicular glutamate transporter 3-like immunoreactivity in the midbrain raphe of Syrian hamsters (Mesocricetus auratus)

Vesicular glutamate transporter 3 (VGLUT3) expression has been specifically localized to brain regions rich in serotonergic cells. It has been suggested that this transporter may contribute to the regulation of extracellular glutamate concentrations via a nonsynaptic mechanism. In this study, we examine the colocalization of vesicular glutamate transporter 3 immunoreactivity with serotonin immunoreactivity in the dorsal and median raphe nuclei of Syrian hamsters. Brain sections from adult hamsters were fluorescently labeled for serotonin-ir and VGLUT3-ir and examined using confocal microscopy. The results indicate that most serotonergic cells of the midbrain raphe also expressed vesicular glutamate transporter 3. In addition, nonserotonergic cells in these brain regions also show immunoreactivity for the transporter. These data confirm previous findings of vesicular glutamate transporter 3 expression in serotonergic and nonserotonergic neurons in rats. These findings suggest that the location of vesicular glutamate transporter 3 may be as much a function of neuroanatomical location as of the neurochemical identity of the expressing neurons.

Developmentally regulated expression of VGLUT3 during early post-natal life

Three subtypes of vesicular glutamate transporters, named VGLUT1-3, accumulate glutamate into synaptic vesicles. In this study, the post-natal expression of VGLUT3 was determined with specific probes and antiserums in the rat brain and compared with that of VGLUT1 and VGLUT2. The expression of VGLUT1 and VGLUT2 increases linearly during post-natal development. In contrast, VGLUT3 developmental pattern appears to have a more or less biphasic profile. A first peak of expression is centered around post-natal day 10 (P10) while the second one is reached in the adult brain. Between P1 and P15, VGLUT3 is observed in the frontal brain (striatum, accumbens, and hippocampus) and in the caudal brain (colliculi, pons and cerebellum). During a second phase extending from P15 to adulthood, the labeling of the caudal brain fades away. The adult pattern is reached at P21. We further analyzed the transient expression of VGLUT3 in the cerebellum and found it to correspond to a temporary expression in Purkinje cells. At P10 VGLUT3 immunoreactivity was present both in the soma and terminals of Purkinje cells (PC), where it colocalized with the vesicular inhibitory amino acid transporter (VIAAT). In agreement with data from the literature [Gillespie, D.C., Kim, G., Kandler, K., 2005. Inhibitory synapses in the developing auditory system are glutamatergic. Nat. Neurosci. 8, 332-338], our results suggest that during the first 2 weeks of post-natal life PC may have the potential to transiently release simultaneously GABA and glutamate.

Vesicular glutamate transporter 1 immunoreactivity in extrinsic and intrinsic innervation of the rat esophagus

Encouraged by the recent finding of vesicular glutamate transporter 2 (VGLUT2) immunoreactivity (-ir) in intraganglionic laminar endings (IGLEs) of the rat esophagus, we investigated also the distribution and co-localization patterns of VGLUT1. Confocal imaging revealed substantial co-localization of VGLUT1-ir with selective markers of IGLEs, i.e., calretinin and VGLUT2, indicating that IGLEs contain both VGLUT1 and VGLUT2 within their synaptic vesicles. Besides IGLEs, we found VGLUT1-ir in both cholinergic and nitrergic myenteric neuronal cell bodies, in fibers of the muscularis mucosae, and in esophageal motor endplates. Skeletal neuromuscular junctions, in contrast, showed no VGLUT1-ir. We also tested for probable co-localization of VGLUT1-ir with markers of extrinsic and intrinsic esophageal innervation and glia. Within the myenteric neuropil we found, besides co-localization of VGLUT1 and substance P, no further co-localization of VGLUT1-ir with any of these markers. In the muscularis mucosae some VGLUT1-ir fibers were shown to contain neuronal nitric oxide synthase (nNOS)-ir. VGLUT1-ir in esophageal motor endplates was partly co-localized with vesicular acetylcholine transporter (VAChT)/choline acetyltransferase (ChAT)-ir, but VGLUT1-ir was also demonstrated in separately terminating fibers at motor endplates co-localized neither with ChAT/VAChT-ir nor with nNOS-ir, suggesting a hitherto unknown glutamatergic enteric co-innervation. Thus, VGLUT1-ir was found in extrinsic as well as intrinsic innervation of the rat esophagus.